The source that follows with the download is written in Perl, which I have no intentions of honoring. It is riddled with regular expressions, and it relies on MD5 hashes to escape certain characters. Something is just wrong about that!

I'm about to hard code a parser for Markdown. What is experience with this?

If you don't have anything meaningful to say about the actual parsing of Markdown, spare me the time. (This might sound harsh, but yes, I'm looking for insight, not a solution, that is, a third-party library).

To help a bit with the answers, regular expressions are meant to identify patterns! NOT to parse an entire grammar. That people consider doing so is foobar.

If you think about Markdown, it's fundamentally based around the concept of paragraphs.

As such, a reasonable approach might be to split the input into paragraphs.

There are many kinds of paragraphs, for example, heading, text, list, blockquote, and code.

The challenge is thus to identify these paragraphs and in what context they occur.

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I ended up doing the same. However, I'm not trying to parse markdown as if it was a formal grammar, because it's clearly not. I applied different regular expressions in a recursive manner. And in several passes. That worked out very well.
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John LeidegrenFeb 14 '10 at 9:39

@JohnLeidegren, any chance other curious users such as myself can see your attempt at parsing markdown?
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jmlopezFeb 27 '13 at 1:57

@jmlopez Sorry, I don't have access to that source any longer, if you need a markdown parser, there is a NuGet package available that can be used. The idea is simple enough though, just apply a series of regular expression in passes, start by paritioning the input in paragraphs then try to identity what kind of paragraph it is, and so on. Finally, parse links and character styles within the paragraphs themselves.
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John LeidegrenFeb 28 '13 at 6:43

2

You should look at Parsedown. It splits text into lines. Then it looks at how these lines start and relate to each other.
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Emanuil RusevSep 16 '13 at 22:22

I released a new parser-based Markdown Java implementation last week, called pegdown.
pegdown uses a PEG parser to first build an abstract syntax tree, which is subsequently written out to HTML. As such it is quite clean and much easier to read, maintain and extend than a regex based approach.
The PEG grammar is based on John MacFarlanes C implementation "peg-markdown".

I'd probably read the syntax specification enough times to know it, and get a feel for how to parse it.

Reading the existing parser code is of course brilliant, both to see what seems to be the main source of complexity, and if any special clever tricks are being used. The use of MD5 checksumming seems a bit weird, but I haven't studied the code enough to understand why it's being done. A comment in a routine called _EscapeSpecialChars() states:

We're replacing each such character with its corresponding MD5 checksum value;
this is likely overkill, but it should prevent us from colliding with the escape
values by accident.

Replacing a single character by a full MD5 does seem extravagant, but perhaps it really makes sense.

Of course, it'd be clever to consider creating a "true" syntax, for a tool such as Flex to get out of the regex bog.

That MD5 thing still bothers me, also the excessive string manipulation has to be slower than any actual decent parser you could write yourself.
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John LeidegrenMar 3 '09 at 7:53

2

Flex is really only half the parser; once you have tokenized the input, you need to determine what the tokens mean. This is what a parser generator is for. There are lots of them. ("Parser combinator", "recursive-descent" and "LALR(1)" are key words to google for.)
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jrockwayMar 3 '09 at 7:58

1

@jrockway: that is true of course, I guess I shrugged and thought "but if he reads up on Flex, he'll find Bison automatically". :) Thanks.
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unwindMar 4 '09 at 6:43

If I was to try to parse markdown (and its extension Markdown extra) I think I would try to use a state machine and parse it one char at a time, linking together some internal structures representing bits of text as I go along then, once all is parsed, generating the output from the objects all stringed together.

Basically, I'd build a mini-DOM-like tree as I read the input file.
To generate an output, I would just traverse the tree and output HTML or anything else (PS, LaTex, RTF,...)

Things that can increase complexity:

The fact that you can mix HTML and markdown, although the rule could be easy to implement: just ignore anything that's between two balanced tags and output it verbatim.

URLs and notes can have their reference at the bottom of the text. Using data structures for hyperlinks could simply record something like:

Regex and perl are best friends because somebody said so. There's no more truth to that fact than it's historical ancestry, that it has been used like that. I have no use for something like perl.
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John LeidegrenMar 3 '09 at 7:55

If you are using a programming language that has more than three other
users, you should be able to find a library to parse it for you. A
quick Google-ing reveals libraries for CL, Haskell, Python,
JavaScript, Ruby, and so on. It is highly unlikely that you will need
to reinvent this wheel.

If you really have to write it from scratch, I recommend writing a
proper parser. With this technique, you won't have to escape things
with MD5 hashes. (I agree that if you have to do something like this,
it's time to reconsider your design.)

I'm up for the challenge. I looked at libraries but they're just awful. Ugly and stupid. I'm considering writing the parser in F# because I need a F# project but I'll probably end up doing it in C#.
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John LeidegrenMar 3 '09 at 7:58

Hopefully F# has a library like Parsec; if so, this will be a fun project ;)
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jrockwayMar 3 '09 at 15:12